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WASHINGTON (AP) - On the eve of their Super Tuesday showdown, Mitt Romney and Rick Santorum strained for an edge in Ohio on Monday and braced for the 10 primaries and caucuses likely to redefine the race for the Republican presidential nomination.

Newt Gingrich, though winless for more than a month, campaigned in Tennessee and issued a stream of signals that he intended to stay in the race.

In a race marked by unpredictability, Romney's superior organization and the support of an especially deep-pocketed super PAC allowed him to compete all across the Super Tuesday landscape and potentially pick up more than half of the 419 delegates at stake.

Santorum cast the race in biblical terms, his David vs. Romney's Goliath. Even that "is probably a little bit of an understatement," he added.

Primaries in Ohio, Georgia, Massachusetts, Vermont, Virginia, Oklahoma and Tennessee plus caucuses in Idaho, North Dakota and Alaska make Tuesday the busiest day of the primary season.

Unlike previous Republican campaigns, when a primary winner would typically win all of a state's delegates, allocations this year generally reflect the split in the popular vote. As a result, several candidates may be able to claim success once the Super Tuesday results are known.

Romney kept his focus on the economy in a final sprint across Ohio, the state that has drawn the most attention and television advertising. Pre-primary polls show him with momentum in a close race with Santorum.

"Other people in this race have debated about the economy, they've read about the economy, they've talked about it in subcommittee hearings," Romney said dismissively of his opponents. "But I've actually been in it. I've worked in business, and I understand what it takes to get a business successful and to thrive."

Santorum, who narrowly lost Michigan to Romney last week, said that no matter how much his rival spends, "conservatives will not trust him, will not rally around him this primary season. ... We will be the nominee."

He said he looked forward to the day when Gingrich drops out and clears the way for him to challenge Romney one-on-one. "And when we do that, we'll win," Santorum said.

Gingrich, the former speaker of the House, was having none of it.

Seemingly confident of a primary victory in Georgia, where he launched his political career more than three decades ago, he unveiled a new television commercial in Tennessee promising to reduce the rising cost of gasoline. Eager to demonstrate his staying power, he said the commercial would soon begin running in Alabama and Mississippi, which hold primaries next week, and he announced a list of supporters in Kansas, where caucuses are on the schedule for Saturday.

The fourth man in the race, Texas Rep. Ron Paul, campaigned in Idaho after a weekend visit to Alaska, where he hopes to claim his first victory of the campaign.

Romney has won four contests in a row, including a double-digit victory in Washington state caucuses on Saturday.

He has 203 delegates in the Associated Press count, while Santorum has 92, Gingrich 33 and Paul 25.

It takes 1,144 to win the nomination at the convention in Tampa, Fla., next summer.

Romney's itinerary on Monday underscored the extent to which the campaign for the nomination has changed from closely watched statewide contests into to an all-out battle for individual delegates. While he hoped to win the Ohio primary outright, he arranged stops in Canton and Youngstown, in and around areas where Santorum isn't eligible for all the delegates available on Tuesday.

Santorum was hampered by his failure to file any delegates in three of the state's 16 congressional districts. That meant he was forfeiting any chance at nine of the 63 at stake, even if he won statewide.

More damaging to their hopes of stopping Romney, Santorum and Gingrich failed to qualify for the Virginia primary ballot, and Romney appeared in line to capture all 46 delegates there. The former Massachusetts governor also has virtually no competition on his home turf in that state, with 38 delegates, and little in Vermont, with 17 more.

Romney pressed his advantage in other ways, from personal endorsements to a huge disparity in television ads across the country.

One day after winning the support of House Majority Leader Eric Cantor of Virginia and Oklahoma Sen. Tom Coburn, he drew backing from former Attorney General John Ashcroft. Romney has long claimed he is the most electable of all the Republicans in the race, and the endorsements were a fresh sign that the party establishment has begun to rally to his side.

While Santorum recently reported taking in $9 million in campaign donations in February, he was outspent across the board on television.

In Ohio, Romney's campaign paid $1 million for television advertisements, and Restore Our Future, which supports him, spent an additional $1.5 million. Santorum and Red, White and Blue, a super PAC that supports Gingrich, countered with about $500,000 combined, according to information on file with the Federal Election Commission, a deficit of approximately 5-1.

In Tennessee, where Romney did not purchase television time, Restore Our Future spent more than $600,000 to help him. Santorum paid for a little over $100,000, and Winning our Future, a super PAC that backs Gingrich, about $470,000.

In Georgia, where Gingrich acknowledged he must win, the pro-Romney super PAC spent nearly $1 million in hopes of holding Gingrich below 50 percent of the vote.

Associated Press writers Kasie Hunt and Steve Peoples in Ohio, Tom Beaumont in Tennessee, Beth Fouhy in New York and Stephen Ohlemacher in Washington contributed to this story.